Multichannel Field Effect Transistor (FET) devices are known in the art. It is known to manufacture multichannel devices pursued using a selective removal of sacrificial epitaxial layers. Commonly these layers are either silicon-germanium (SiGe) or epitaxially grown silicon (Si epi) layers that can be etched isotropically and with high selectivity relative to Si or SiGe, respectively. The use of etchants that selectively etch SiGe with respect to Si in the fabrication of multichannel devices is described in Applicant's previous patents U.S. Pat. Nos. 6,921,700 and 7,112,832.
Also the SON (silicon on nothing) approach (see, for example, S. Monfray et al., IEDM 2004, 27.3.1-27.3.4 (635-638)) relies on a selective removal of SiGe epi layers relative to crystalline Si. In the last two years, the SON approach has been extended to multichannel devices.
However, the present inventors have now observed that proceeding according to currently known techniques for selective etching of SiGe layers in multiple stacked alternating —Si—[(SiGe)—Si]u— layers (u=2 or more) leads to non-uniformity of selectively etched SiGe layers. This results in uncontrollable devices and dysfunctional circuits.
The situation according to present methods is illustrated schematically by FIGS. 1 and 2. FIG. 1 shows a known precursor of a multichannel device containing SiGe layers intercalated in between SiGe layers, the SiGe layers all having the same composition. This is described in Applicant's previous patents U.S. Pat. Nos. 6,921,700 and 7,112,832.
As shown schematically in FIG. 2 and through transmission electron microscopy (TEM) evidence in FIGS. 3 to 6, where the SiGe layers all have the same composition, non-uniformity in the lateral extent of SiGe etching is observed.
Without wishing to be bound by any particular theory, it is currently postulated by the inventors that the etch non-uniformity of SiGe layers at a constant Ge content is due to non-uniform stress distribution in the layer stack.